void setup() {
  size(200,200);
  framerate(20);
}

void loop() {
  proc();
  drawproc();
}
Spin3d spin = new Spin3d();

void mouseDragged() {
  spin.mouseMoved();
}

float rx = 0;
void proc() {
  rx+=0.1;
  spin.update();
}

boolean showStencil = false;

void drawproc() {
  translate(width/2,height/2);
    
  background(255);
  push();
    translate(0,0,-50);
    rotateX(-PI/4+rx*0.05);
    smooth();
    noStroke();
    drawScene(true);
    for (int i=0; i<ax.length; i++) {
      au[i] = screenX(ax[i], ay[i], az[i]);
      av[i] = screenY(ax[i], ay[i], az[i]);
    }
    noSmooth();
  pop();

  if (keyPressed&&key=='s') return;  
  
  BImage t = copy();
  
  background(128);
  
  translate(0,0,-100);
  spin.apply();
  
  stroke(0);
  drawScene(false);
  
  noStroke();
  fill(255);
  beginShape(QUAD_STRIP);
    texture(t);
    for (int i=0; i<ax.length; i++)
      vertex(ax[i], ay[i], az[i], au[i], av[i]);
  endShape();
}
int s = 100;
float ax[] = {-s,  s,  s, -s, -s, s};
float ay[] = {-s, -s,  s,  s,  s, s};
float az[] = {-s, -s, -s, -s,  s, s};
float au[] = new float[ax.length];
float av[] = new float[ax.length];

void drawScene(boolean shadow) {
  push();
    fill(shadow ? 0 : 200);
    rotateZ(rx);
    rotateX(rx*0.5);
    
    push();
      scale(2,0.5,0.5);
      box(50);
    pop();
    push();
      scale(0.5,2,0.5);
      box(50);
    pop();
    push();
      scale(0.5,0.5,2);
      box(50);
    pop();
    
  pop();
}

public class Spin3d {
  float x= 0, y = 0;
  float tdx = 0, tdy = 0;
  float dx = tdx, dy = tdy;


  public void mouseMoved() {
    // GOTCHA: x and y are switched!
    tdx=0.01*(mouseY-pmouseY);
    tdy=0.01*(mouseX-pmouseX);
  }
  public void apply() {
    g.transform(m_0_0, m_0_1, m_0_2, m_0_3,
                m_1_0, m_1_1, m_1_2, m_1_3,
                m_2_0, m_2_1, m_2_2, m_2_3,
                m_3_0, m_3_1, m_3_2, m_3_3);
  }
  void update() {
    x += dx;
    y += dy;
    dx += (tdx-dx)/20;
    dy += (tdy-dy)/20;
    rotateX(dx);
    rotateY(dy);
  }

	public Spin3d() {
	}
	public float	
		m_0_0 = 1, m_0_1 = 0, m_0_2 = 0, m_0_3 = 0,
		m_1_0 = 0, m_1_1 = 1, m_1_2 = 0, m_1_3 = 0,
		m_2_0 = 0, m_2_1 = 0, m_2_2 = 1, m_2_3 = 0,
		m_3_0 = 0, m_3_1 = 0, m_3_2 = 0, m_3_3 = 1;

	public void rotateX(float t) {
		if (t==0) return;
		float sin = sin(t);
		float cos = cos(t);
		transform(
			1,    0,   0,   0,
			0,    cos, sin, 0,
			0,    -sin,cos, 0,
			0,    0,   0,   1
		);
	}
	public void rotateY(float t) {
		if (t==0) return;
		float sin = sin(t);
		float cos = cos(t);
		transform(
			cos, 0,   sin, 0,
			0,   1,   0,   0,
			-sin,0,   cos, 0,
			0,   0,   0,   1
		);
	}
	
	public void transform(float nm_0_0, float nm_0_1, float nm_0_2, float nm_0_3,
						float nm_1_0, float nm_1_1, float nm_1_2, float nm_1_3,
						float nm_2_0, float nm_2_1, float nm_2_2, float nm_2_3,
						float nm_3_0, float nm_3_1, float nm_3_2, float nm_3_3) {
							
		Spin3d m =  new Spin3d();

		m.m_0_0 = m_0_0; m.m_0_1 = m_0_1; m.m_0_2 = m_0_2; m.m_0_3 = m_0_3;
		m.m_1_0 = m_1_0; m.m_1_1 = m_1_1; m.m_1_2 = m_1_2; m.m_1_3 = m_1_3;
		m.m_2_0 = m_2_0; m.m_2_1 = m_2_1; m.m_2_2 = m_2_2; m.m_2_3 = m_2_3;
		m.m_3_0 = m_3_0; m.m_3_1 = m_3_1; m.m_3_2 = m_3_2; m.m_3_3 = m_3_3;

		m_0_0 = nm_0_0*m.m_0_0 + nm_0_1*m.m_1_0 + nm_0_2*m.m_2_0;
		m_0_1 = nm_0_0*m.m_0_1 + nm_0_1*m.m_1_1 + nm_0_2*m.m_2_1;
		m_0_2 = nm_0_0*m.m_0_2 + nm_0_1*m.m_1_2 + nm_0_2*m.m_2_2;
		m_0_3 = nm_0_0*m.m_0_3 + nm_0_1*m.m_1_3 + nm_0_2*m.m_2_3 + nm_0_3;
		m_1_0 = nm_1_0*m.m_0_0 + nm_1_1*m.m_1_0 + nm_1_2*m.m_2_0;
		m_1_1 = nm_1_0*m.m_0_1 + nm_1_1*m.m_1_1 + nm_1_2*m.m_2_1;
		m_1_2 = nm_1_0*m.m_0_2 + nm_1_1*m.m_1_2 + nm_1_2*m.m_2_2;
		m_1_3 = nm_1_0*m.m_0_3 + nm_1_1*m.m_1_3 + nm_1_2*m.m_2_3 + nm_1_3;
		m_2_0 = nm_2_0*m.m_0_0 + nm_2_1*m.m_1_0 + nm_2_2*m.m_2_0;
		m_2_1 = nm_2_0*m.m_0_1 + nm_2_1*m.m_1_1 + nm_2_2*m.m_2_1;
		m_2_2 = nm_2_0*m.m_0_2 + nm_2_1*m.m_1_2 + nm_2_2*m.m_2_2;
		m_2_3 = nm_2_0*m.m_0_3 + nm_2_1*m.m_1_3 + nm_2_2*m.m_2_3 + nm_2_3;
	}
}